There is an active interest in the production of industrial feedstock chemicals or fuels from biomass. Starch is a major component of agricultural crops and of corn processing waste and it is a preferred substrate for chemical and enzyme production because of its chemical composition and its higher density than other forms of biomass which facilitates prolonged storage, and decreases transportation and pretreatment costs.
Starch is known to be a valuable starting material for the enzymatic production of sugar, such as glucose, which may be converted by yeast to ethanol. The main amylolytic or starch converting enzymes used for the industrial production of glucose, maltose and maltosaccharide from starch are .alpha.-amylase, .beta.-amylase, glucoamylase and pullulanase.
The known amylolytic enzymes, except for bacterial .alpha.-amylases, are unstable at the elevated temperatures preferred for the industrial conversion of starch.
In our earlier U.S. patent application Ser. Nos. 652,585; 652,586 and 652,588, all filed Sept. 18, 1984 we disclosed the preparation of thermostable .beta.-amylase; the preparation of thermostable glucoamylase and pullulanase and the co-culture production of those enzymes and ethanol, respectively.
Although the microbial strains disclosed in our earlier applications produce the thermostable enzymes in useful quantities, it would obviously be desirable to have mutant strains that result in a more efficient method of converting starch into ethanol and greater production of thermostable enzymes.